Continuing the quest of Mohorovičić

Most people are quite content with an annual holiday abroad, yet a number of geoscientists yearn for something more adventurous. The Croatian geophysicist Andrija Mohorovičić was among the first to study estimates of speeds at which seismic waves travelled through the Earth, discovering in 1909 that below a depth of about 30 km below the continental surface they moved faster than in the uppermost layer. He had discovered the boundary between the continental crust and the underlying mantle, a discontinuity that bears his name though often shortened to the ‘Moho’. Having been traced beneath most of the Earth’s surface, a group of American scientists discussed over a drink or three at a ‘wine breakfast’ in 1957 a project to drill through the Moho to find out what the mantle was made of. The brainchild of Harry Hess, one of the first to suggest plate tectonics as a driving mechanism for continental drift, was dubbed Project Mohole. With US government support, a drilling barge designed for offshore oil drilling and a system of thrusters and pre-GPS locational instrumentation to keep the barge on station the Mohole was spudded in 1961 on the seabed near Guadalupe Island off Baha California in Mexico; about the time that John F. Kennedy declared his belief that the USA could land a man on the Moon by the end of the 1960s. There was something of a thrill factor about Project Mohole, and its first attempts were reported in Life Magazine by John Steinbeck, author of The Grapes of Wrath and amateur oceanographer. It turned out that sending a drill bit to the mantle was more difficult than a manned lunar landing. Only a few metres of basaltic crust was recovered and Congress cancelled Mohole funding in 1966. Nevertheless, the project was the forerunner of the highly successful Ocean Drilling Program and its predecessors, probably the most prolific international collaboration of any kind.

The drilling barge CUSS1 used for the original Mohole Project. Image via Wikipedia

Since the 1960s research into the mantle has been continued with great success by looking at upthrust masses such as those in the Alps and in ophiolite complexes, nodules in alkaline basalts and kimberlites that form below 100 km into the mantle, samples dredged from oceanic fracture zones, and indirectly from the geochemistry of basalts that are derived by partial melting of mantle materials. Yet, there is still an air of frustration about some igneous petrologists and geophysicists; they want to touch the real thing! Now, at last, they may have their chance, for improved drilling and positioning technology developed by ODP and the petroleum industry make a hole through the Moho feasible. Indeed one is planned once drill-bits and lubricants suitable for the anticipated temperatures and pressures have been finalised. Three sites are under consideration: near the original Mohole; in the Cocos Plate off Costa Rica and the Pacific Plate near Hawaii, each combining the coolest crust, thinnest sediment cover and shallowest possible water – i.e. just off a mid-ocean ridge or hot-spot. The Costa Rica site (ODP site 1256) has the thinnest crust due to rapid sea-floor spreading by the East Pacific Rise there and is the most likely to be drilled. It already has a core the penetrates to 1.5 km in oceanic crust and a current project aimed at sampling the cumulate gabbro layer of the lower oceanic crust. That will still be 3.5 km above the local Moho.

There is an obvious question; will an ocean-floor site, however favourable, and a hole drilled through it help resolve fundamental issues regarding the mantle? Well, probably for oceanic lithospheric mantle, but that has had basaltic magma removed from it to form the crust above. Also mid-ocean ridge basalts have geochemical features that suggest that their source mantle had been a melt source previously, compared with the source mantle materials for alkaline and some other types of basalt that seem to have been less depleted in certain elements. The most important question posed by the mantle in general concerns how it originally formed during the Earth’s earliest history, accretion of debris from the solar nebula, the moon-forming event and extraction of the metallic core. A Mohole can contribute little to those issues.

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